Radio receiver



Sept. 14, 1937. q, s, STARRETT $093,102

RADIO RECEIVER Filed Jan. -7, 1955 "1 l l l. l ml l l 1 i i E INVENTOR. N J JOHN S. STARRETT ATTORNEY.

Patented Sept. 14, 1937 RADIO RECEIVER John s. Starrett, Wilmette, 111., assignor to Radio Corporation of America, a corporation of Delaware Application January 7, 1935, Serial No. 629

6 Claims.

An object of this invention is to provide a receiver for audio modulated radio frequency currents which is provided with a single tube which serves as a combined detector and power am- 5 plifier and having sufficient power output to directly operate a loud speaker or other sound translating device.

A further object .of the invention is to provide a diode detector having two co-planar control grids which are operated at aphase differ,- ence of substantially with respect to the applied radio frequency voltage but in phase with each other with respect to the modulation frequency.

A still further object is to provide a double diode detector and amplifier having a screen grid and also a suppressor grid and in which connections are so made that the voltage of these latter elements is automatically controlled to limit the plate current to a predetermined safe value and thus prevent overloading of the tube when the applied signal voltage falls off to a low value or even to zero.

For a better understanding of the invention reference is made to the accompanying drawing and specification in which a single embodiment thereof is shown, but it will be understood that the invention is not limited to the specific arrangement disclosed but is intended to include all forms and embodiments which come within thescope of the following claims.

Referring to the single figure of the drawing the incoming signal currents as picked up on the antenna l are amplified in the radio frequency amplifier tubes 5 and 6 each of which has an input circuit 2 which is tunable to the desired sig nal frequency by the tuning condensers 3 which are of similar construction and may be operated by a single control shaft as indicated by the reference number 4. .The output circuit of tube 6 is coupled by the radio frequency transformer l to the input circuit of the double diode detector and audio tube l4. As shown, the secondary planar grids.

As shown, a midpoint of secondary coil 8 is connected to the cathode l5 of the I tube through the resistor 25 which is shunted by thecondenser 26. A contact 28 is adjustable along resistor 25 and isconnected to cathode [5 through the condenser 21. ode I5 is of the indirectly heated type being heated by the filament 29 but it will be understood that a directly heated cathode may be used if desired. The cathode I5 is also connected to ground through the bias resistor 23 which is shunted by a condenser 25. In the electron stream between the cathode and plate I3 the tube alsois provided with a screen grid H and a suppressor grid l2, the latter being connected to ground through the lead it, the grid l I being connected to a steady source of positive potential through a resistor 2|, the upper end of which is connected to ground through a condenser 22. A suitable operating potential is applied to the plate l3 through the primary winding of an audio frequency transformer H which may be connected in series with a resistor I9 to the positive terminal of a voltage source such as the battery 30, the negative terminal of which is grounded. The low potential end of resistor I9 is shown connected to ground through the b-y-pass condenser 20. 1 The secondary winding of the output transformer I1 is shown connected directly to the terminals of the loud speaker l8.

Inthe operation of the device the incoming signal .currents after being amplified in the amplifier stages 5 and 6 cause one of the co-planar grids 9 to be driven positive while the other grid it] is driven negative by the same amount and therefore, combined, action of these two grids does not change the current flowing between plate l3 and cathode l5 at radio frequency. However, by virtue of the detection action of grid leak 25 and condenser 26 the voltage of each of these grids will vary in phase at the modulation frequency of the signal. By virtue of the amplifying property of the tube the modulation frequency current will appear in amplified form in the plate circuit and be impressed through the transformer I! on the loud speaker. When signals of normal amplitude are impressed on the secondary 8 by Virtue of the direct current drop through the resistor 25 the grids 9 and Ill assumea normal negative bias with respect to the cathode l5 thereby causing the normal plate current to fiow between plate I3 and cathode l5 for which the tube is designed. If now the signal voltage applied to secondary 8 is substantially reduced due to'the voltage of grids 9 and I0 increasing and approaching that of cathode Hi, the current pass- As shown, the cathing through the tube between plate l3 and cathode l5 might be increased to an undesirable amount causing over-loading of the tube and consequent distortion and possibly burning out of the tube. In my arrangement this action is prevented by the resistors 2| and 23 and by connecting the suppressor grid [2 to ground whereby it is given a. negative bias with respect to the cathode. Upon an increase in potential of grids 9 and H] the screen current to grid ll would increase, thereby causing the voltage drop through resistor 2! to increase thus decreasing the potential of grid ll. would assist in decreasing the plate current to a normal value. This compensating action is further assisted by the grid l2, whose bias voltage is increased upon an increase of the direct plate current through resistor 23, so that as grid l2 becomes more negative it also will assist in decreasing the plate current.

The tube is designed to operate with a normal bias when sufficient signal voltage is applied about substantially the midpoint of the control grid-plate current characteristic so as-to provide for normal modulation peaks swinging on the straight portion of the-curve. Also at normal modulation levels sufiicient audio voltage would be developed across resistor 25 to give optimum output when from one quarter to one half of resistor 25 is shunted by condenser 21, thereby allowing the shunting effect to be reduced, when asignal is received'with a-sumcient amplitude for normal operation but having a low per cent modulation. Condensers 22 and 24 are chosen of sufficient capacity to prevent audio components from building up across resistors 2| and 23.

I have found the following circuit values to provide proper operation of a tube of the type described: Condenser 21 may have a value of 5 to 10 microfarads, resistor 25:0ne half megohm, condenser 26:.00025 microfarad, resistor 2|: about 50,000 ohms, condensers 22 and 24 have values of about 1 microfarad.

While I have shown the invention embodied in a receiver ofthe tuned radio frequency type it will be understood that it may be employed equally well in one of the superheterodyne' type such, for example, as that described in Armstrong Patent No. 1,342,885, in which case the secondary coil 8 may be tuned to the intermediate frequency of the receiver by means of a condenser connected across its ends.

Having described my invention, what I claim as novel and desire to secure by Letters Patent is:

1. In combination, a detector having a cathode and input electrode, an input circuit connectingsaid electrode and cathode including a resistor, a condenser shunted across said re- This decreased potential' sistor, a contact adjustable along said resistor, a second condenser having one terminal connected to said adjustable contact and another terminal directly connected to said cathode and means for impressing modulated radio frequency currents on said input circuit.

2. The combination defined in the preceding claim in which said second condenser has only a small impedance value for currents of audio frequency.

3. In a detector arrangement, the combination of a vacuum tube having a cathode, two grids and a plate, a transformer having primary and secondary windings, the terminals of said secondary Winding being directly connected to said grids, a circuit connecting the midpoint of said secondary and said cathode and including a re- I sistor, a condenser connected across the ends of said resistor, a second condenser having one terminal connected to an intermediate point on said resistor and another terminal connected directly to said cathode said second condenser having a low impedance to currents of audio frequency, and an output circuit connected to said plate.

4. The combination defined in the preceding claim in which said tube has an auxiliary grid located adjacent said plate and means for caussaid co-planar grids, a load circuit comprising.

a steady source of potential having its positive terminal connected to said plate, a resistor having one end connected to the negative terminal of said potential source and its other end to said cathode, a condenser shunted across said resistor, a conductive connection between said suppressor grid and the negative terminal of said potential source and a resistor having one end connected to said screen grid and its other end to a positive source of potential.

6. The combination defined in the preceding claim in which a loud speaker is coupled to said load circuit the average potentials of said screen and suppressor grids being chosen to cause sufficient power output in the load circuit to drive the loud speaker when a signal voltage of normal amplitude is applied to theprimary of said transformer.

' JOHN S. 'STARRET'I. 

